1. Field of the Invention
The present invention relates to a magnetoresistive (MR) effect element that provides an output based on resistance change according to the intensity of a signal magnetic field, a thin-film magnetic head including the MR effect element, a head gimbal assembly (HGA) provided with the thin-film magnetic head, and a magnetic recording/reproducing apparatus provided with the HGA.
2. Description of the Related Art
As magnetic recording/reproducing apparatuses as represented by magnetic disk drive apparatuses increase in capacity and reduce in size, thin-film magnetic heads are required to have higher sensitivity and larger output. To respond to the requirement, a giant magnetoresistive (GMR) effect element and a tunnel magnetoresistive (TMR) effect element have been developed, which can detect extremely local signal magnetic field and provide significantly high resistance-change ratio. Actually, in thin-film magnetic heads having the element as a read head element for reading data, output characteristics of the heads are being intensively developed.
The MR effect elements such as the above-described TMR and GMR effect elements have a magnetization-pinned layer (pinned layer) and a magnetization-free layer (free layer) which are two ferromagnetic layers opposed to each other so as to sandwich a non-magnetic intermediate layer. The magnetization direction of the pinned layer is fixed due to the exchange coupling with an antiferromagnetic layer having a surface contact with the pinned layer on the opposite side to the non-magnetic intermediate layer. On the other hand, the magnetization direction of the free layer can change according to a signal magnetic field generated from a magnetic recording medium. In this configuration, the signal magnetic field is detected by measuring the element resistance variation as a function of the magnetization direction of the free layer.
Therefore, in developing the element output characteristics such as an output intensity and the symmetry of output waveform, it has been one of most significant problems to appropriately control the magnetization directions of respective constituent layers above-described.
Generally, as for the magnetization direction of the free layer, it is important to apply an appropriate bias magnetic field to the free layer so that the magnetic domains of the free layer is stabilized and an output responding linearly to the signal magnetic field is obtained. As a method for applying the bias magnetic field, generally used is an abutted junction biasing method. In the method, bias layers formed of a hard-magnetic material are disposed near both ends in the track width direction of the free layer, and then a bias magnetic field in the track width direction is applied to the free layer. The abutted junction biasing method can cause the magnetic domains of the free layer to be stabilized effectively because both end portions in the track width direction of the free layer, which are most affected by demagnetizing field, can receive most amount of bias magnetic field.
In the meanwhile, the magnetization of the pinned layer is usually fixed to the direction in-plane of the pinned layer and perpendicular to the track width direction through a pin-annealing process. When the magnetization of the pinned layer is rotated to be tilted from the just perpendicular direction or is dispersed, a problem is likely to occur that sufficient element output cannot be obtained or that the symmetry of the output waveform is degraded. Against the problem, conventionally, many measures with respect to the pin-annealing process have been devised, as described, for example, in Japanese Patent Publication No. 2005-56538A.
However, even if the magnetization of the pinned layer was fixed by a predetermined pin-annealing process, there has been a problem that the magnetization may be rotated or dispersed by annealing afterward under the condition of narrower read gap required for the recent higher density recording. Here, the read gap is defined as a distance between two shield layers sandwiching a magneto-sensitive portion of the MR effect element. The read gap tends to be narrower so that an extremely local signal magnetic field can be detected.
Actually, in the wafer thin-film process of the head manufacture, an electromagnetic coil element for writing data is usually formed after forming the MR effect element. The formation process of the electromagnetic coil element involves an annealing step with significantly high temperature. Further, in the machine process in which the wafer substrate that has gone through the wafer thin-film process is separated into individual sliders, various annealing processes are performed. The high temperature environment in these annealing processes causes the exchange coupling between the antiferromagnetic layer and the pinned layer to be weakened. The decrease in the coupling force becomes more significant especially when the thickness of the antiferromagnetic layer becomes smaller according to narrowing the read gap. As a result, even if the magnetization of the pinned layer was fixed by a predetermined pin-annealing process, there has been a case that the magnetization is rotated or dispersed through the annealing afterward, so that the element output decreases and the symmetry of the output waveform is degraded.